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<a name="Debug-Information"></a>
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<a name="Variable-Location-Debug-Information-in-RTL"></a>
<h3 class="section">14.18 Variable Location Debug Information in RTL</h3>
<a name="index-Variable-Location-Debug-Information-in-RTL"></a>

<p>Variable tracking relies on <code>MEM_EXPR</code> and <code>REG_EXPR</code>
annotations to determine what user variables memory and register
references refer to.
</p>
<p>Variable tracking at assignments uses these notes only when they refer
to variables that live at fixed locations (e.g., addressable
variables, global non-automatic variables).  For variables whose
location may vary, it relies on the following types of notes.
</p>
<dl compact="compact">
<dd><a name="index-var_005flocation"></a>
</dd>
<dt><code>(var_location:<var>mode</var> <var>var</var> <var>exp</var> <var>stat</var>)</code></dt>
<dd><p>Binds variable <code>var</code>, a tree, to value <var>exp</var>, an RTL
expression.  It appears only in <code>NOTE_INSN_VAR_LOCATION</code> and
<code>DEBUG_INSN</code>s, with slightly different meanings.  <var>mode</var>, if
present, represents the mode of <var>exp</var>, which is useful if it is a
modeless expression.  <var>stat</var> is only meaningful in notes,
indicating whether the variable is known to be initialized or
uninitialized.
</p>
<a name="index-debug_005fexpr"></a>
</dd>
<dt><code>(debug_expr:<var>mode</var> <var>decl</var>)</code></dt>
<dd><p>Stands for the value bound to the <code>DEBUG_EXPR_DECL</code> <var>decl</var>,
that points back to it, within value expressions in
<code>VAR_LOCATION</code> nodes.
</p>
<a name="index-debug_005fimplicit_005fptr"></a>
</dd>
<dt><code>(debug_implicit_ptr:<var>mode</var> <var>decl</var>)</code></dt>
<dd><p>Stands for the location of a <var>decl</var> that is no longer addressable.
</p>
<a name="index-entry_005fvalue"></a>
</dd>
<dt><code>(entry_value:<var>mode</var> <var>decl</var>)</code></dt>
<dd><p>Stands for the value a <var>decl</var> had at the entry point of the
containing function.
</p>
<a name="index-debug_005fparameter_005fref"></a>
</dd>
<dt><code>(debug_parameter_ref:<var>mode</var> <var>decl</var>)</code></dt>
<dd><p>Refers to a parameter that was completely optimized out.
</p>
<a name="index-debug_005fmarker"></a>
</dd>
<dt><code>(debug_marker:<var>mode</var>)</code></dt>
<dd><p>Marks a program location.  With <code>VOIDmode</code>, it stands for the
beginning of a statement, a recommended inspection point logically after
all prior side effects, and before any subsequent side effects.  With
<code>BLKmode</code>, it indicates an inline entry point: the lexical block
encoded in the <code>INSN_LOCATION</code> is the enclosing block that encloses
the inlined function.
</p>
</dd>
</dl>

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